MG mechanic says...

[Eye of the Tiger]

Well, the idea is that since the HCs at idle are the only thing that's really high, it's a misfire at idle. The vacuum leak, uh....

Brain died.

Can you just adjust your idle mixture, then? Or did Mr. MG mechanic already do that?

[Rupert]

I could lean it, but that wouldn't do enough. With the HCs that high, and the CO being OK, there is only so much I could lean it before one of those went out of spec. The HCs are from unburned hydrocarbons (gas or oil). The diagnosis with just the very high HC and OK CO is a misfire. The carbs are tuned right, though if everything was too high, I could lean it out a bit.

[S204STi]

Well, the idea is that since the HCs at idle are the only thing that's really high, it's a misfire at idle. The vacuum leak, uh....

Brain died.

Like I said, carbs are foriegn to me, but in an EFI setup a vacuum leak would cause a lean misfire because you are getting more air than fuel and the computer can't see that to compensate because it's happening after the MAF or whatever speed density sensors it uses.  With a carb you basically set it to run at a certain a/f ratio, right?  Well I suppose if you had a vacuum leak after the carb that would screw with your adjustments, maybe causing you to go too rich with it to compensate, so I guess I could see that happening with your car.

[S204STi]

I could lean it, but that wouldn't do enough. With the HCs that high, and the CO being OK, there is only so much I could lean it before one of those went out of spec. The HCs are from unburned hydrocarbons (gas or oil). The diagnosis with just the very high HC and OK CO is a misfire. The carbs are tuned right, though if everything was too high, I could lean it out a bit.

I forget, did you already rebuild your carbs?

[Rupert]

Nope. I don't think they need it. I might do it anyway, eventually. The kit is cheap enough, so it's just a matter of me being smart enough.

[Rupert]

Like I said, carbs are foriegn to me, but in an EFI setup a vacuum leak would cause a lean misfire because you are getting more air than fuel and the computer can't see that to compensate because it's happening after the MAF or whatever speed density sensors it uses.  With a carb you basically set it to run at a certain a/f ratio, right?  Well I suppose if you had a vacuum leak after the carb that would screw with your adjustments, maybe causing you to go too rich with it to compensate, so I guess I could see that happening with your car.

Hmm, yeah. I thought I understood why a vacuum leak would cause high HC, but it turns out I don't. I've seen it from more than one source, though, so it must be true ( ).

Oh oh oh! OK, so when I tuned the mixture, the front carb was really lean, which I thought was odd, because my plugs were fouling. So, if there's a vacuum leak after the front carb, then that would lean the mixture. So really, I was just richening up the carb too much, when I should have fixed the leak. But then why are the HCs fine at 2500 rpm? Would that be because the vacuum leak is stronger at higher RPMs? But I tuned the carbs at idle (duh)...

It's a theory, at least.

[S204STi]

Hmm, yeah. I thought I understood why a vacuum leak would cause high HC, but it turns out I don't. I've seen it from more than one source, though, so it must be true ( ).

Oh oh oh! OK, so when I tuned the mixture, the front carb was really lean, which I thought was odd, because my plugs were fouling. So, if there's a vacuum leak after the front carb, then that would lean the mixture. So really, I was just richening up the carb too much, when I should have fixed the leak. But then why are the HCs fine at 2500 rpm? Would that be because the vacuum leak is stronger at higher RPMs? But I tuned the carbs at idle (duh)...

It's a theory, at least.

Sometimes higher rpms can mask a leak.

[Rupert]

Well, there we go, then. I guess I'll put that to the test after I get the headlight relays installed. The wiring is buried under a horn, and it's all old and dirty. What a pain in the ass.